Although the functioning of river floodplains as sink or source of nutrients has been studied extensively for temperate regions, similar studies in tropical regions are less abundant and studies integrating data about floodplain soil, vegetation, and water are scarce. We examined and compared nutrient contents in soil, water, and vegetation tissue in two different vegetation zones on the monsoon Songkhram river floodplain (Thailand). Significant differences were found between bamboo and grass zones. The soil in the bamboo zone is more fertile than the soil in the grass zone, as indicated by the lower C/N ratio, and has significantly higher organic matter and higher total N and K. Bamboo leaf tissue had significantly higher concentrations of nutrients than grass biomass. The growth of the bamboo is P-limited or P and N co-limited, but grass is N-limited. In both zones, the soil-available P and organic carbons after flooding were significantly lower than before flooding. Floodwater in both zones had low dissolved solid concentrations. After the flood peak, most concentrations tended to increase, especially organic carbon and dissolved nitrogen but phosphorus decreased. The results suggest a significant loss of organic carbon from the soil after flooding, indicating that the floodplain acts as a source of carbon that is exported downstream. Nonetheless it is also evident that the floodwater brings in sediment and nutrients. Based on rough estimations of nutrient budgets we conclude that the highly productive bamboo zone adjacent to the river filters out the nutrients before they reach the grass zone.